Nylon-carbon black composition and article



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United States Patent O NYLON-CARBON BLACK COMPOSITION AND ARTICLE LouisL. Stott, Wyomissing, Pa., assgnor to The Polymer Corporation, acorporation of Pennsylvania Application September 21, 1954, Serial No.457,390

4 Claims. (Cl. 260-37) This invention relates to the art of molding highmolecular weight synthetic linear polyamides and is acontinuation-in-part of applications SerialNo. 227,283 filed May 19,1951 in the name of Louis L. Stott, now abandoned, and Serial No.272,966, tiled February 23, 1952, now Patent 2,695,425, issued November30, 1954, in the name of Louis L. Stott. The synthetic polymericmaterials used in the practice of this invention are the syntheticlinear polyamides of the general type described in the United StatesPatents 2,071,250, 2,071,254 and 2,130,948. The polymers there describedare high molecular weight products which generally can be obtainedcrystalline in structure as evidenced by X-ray powder diffractionpatterns of the polymers in the massive state. t

The polyamides of the present type, generally speaking, comprise thereaction product of a linear polymer-forming composition, for example,one consisting essentially of bifunctional reacting material, whichcomprises in substantial amount molecules containing two amide-forminggroups each of which is complementary to an amideforming group in othermolecules in said composition.

These polyamides as described above, or as otherwise identifiedhereinafter, can be obtained, for example, by self-polymerization ofmonoamino-monocarboxylic acid, or by reacting a diamine with a dibasiccarboxylic acid in substantially equimolecular amounts, it beingunderstood that reference herein to the amino acids, diamines, anddibasic carboxylic acids, is intended to include the equivalentamide-forming derivatives of these reactants.

These linear polyamides include also polymers obtained by admixture ofother linear polymer-forming reactants, as for instance glycol-dibasicacid mixtures in the case of polyester-amides,` with the mentionedpolyamideforming reactants. The best results in the practice of theinvention described herein, however, are obtained with unmodifiedstraight polyamides. In the interpolymers, as well as in the simplepolyamides, the average number of carbon atoms separating the amidegroups is at least two. On hydrolysis with hydrochloric acid, the aminoacid polymers yield the `amino acid hydrochloride, and thediamine-dibasic acid polymers yield the diamine hydrochloride and thedibasic carboxylic acid. In any case, the polyamides are limited tothose which are soluble in phenol at room temperature and are insolublein ethylene glycol except at temperatures above about 140 C. For thesake of simplicity the linear polyamides described above will bereferred to herein as nylon.

Although these materials were originally introduced as fibers for use inthe textile industry, they have subsequently been-made available as rawmaterials for the molding and extrusion industries. The high meltingnylons, such as polyhexamethylene adipamide, and polyhexamethylenesebacamide, are characterized by relatively sharp melting points andhigh fluidity in the molten condition in comparison to otherthermoplastic materials, such as cellulose acetate and polystyrene.These characteristics have made compression molding of these nylonsdicult. At the present time shaped nylon pieces are ICC preparedcommercially, either vby machining solid nylon, such as nylon rod, or bymelting nylon and forming it by injection molding. Both of these methodshave certain drawbacks. Bearings machined from nylon rod, for instance,are relatively expensive to make and involve much Waste. Furthermore,the manufacturing techniques for producing nylon rod, particularly forthe larger sizes, usually introduce severe strains which must beremovedby conditioning. The injection molding technique requirescomplicated and expensive apparatus, high cost molds, and alsofrequently results in a product having many strains. These strains causebearings prepared by injection molding to seize readily if largeclearances are not provided. The presently used molding techniquesdepend upon heating a polyamide above its melting point and exertingpressure on the molten material. The strains produced in the resultantarticle come as a consequence of cooling the molten material and, atleast in part, are due to a relatively high volumetric shrinkage onsolidilication.

In accordance with my United States application Serial No. 227,283,tiled May 19, 1951, molded nylon articles containing fillers aredescribed. These articles are made by compressing nylon powder having anultimate particle size of less4 than 40 microns and a finely dividedfiller with sufficient pressure so that they may be handled. Thecompressed article is thereafter sintered by heating to a temperaturejust below the melting point ofthe nylon. Such fillers are capable ofimparting special desirable characteristics to nylon molded articles.Among such general characteristics are increased dimensional stabilityand decreased lsensitivity to humidity.

It is technically extremely difficult Ito get a uniform dispersion ofcarbon black in molten nylon, particularly where the filler is desiredin high concentrations. For example, such attempts to incorporate l5 or20 percent carbon black with nylon by 'the conventional method of mixingthe carbon black with molding powder is unsatisfactory. When carbonblack and the molding powder are thus mixed and the mixture is heated,poor distribution and -a tendency for the filler to settle result. Ifthe ller is added to molten nylon, segregation of the filler occurs dueto diiference in specific gravity.

It is an object of this invention to produce powder and molded articlesof modified nylon, i. e. nylon having a filler which imparts physicalcharacteristics which diler from those of usual nylon articles. Anotherobject is to prepare molded nylon articles having greater dimensionalstability during humidity changes than unmodified nylon articles. Stillanother object is to produce nylon powder and articles having carbonblack evenly distributed throughout the finished article. Carbon blackbeing of low density is relatively inexpensive on a volumetric basis. Astill further object is to produce a nylon powder mixture having arelatively low fill ratio, which is particularly important to avoid thenecessity of using special press equipment with extra long stroke. Theseand other objects which will become apparent hereafter may beaccomplished as follows:

Nylon powder having an ultimate particle size of less than 40 microns ismixed thoroughly with carbon black and compressed to the desired shapewith suicient pressure so that the shaped green article may be handled.The compressed article is thereafter heated to a temperature sufficientto sinter the particles of nylon together without inducing anysubstantial molten phase. The article may be annealed to remove anyresidual strains.

It is essential that a substantial portion of the nylon particles bebelow about 40 microns in diameter and preferably vbelow 25 microns, butthe optimum size appears to be l0 microns or less. Nylon of this sizemay be obtained in accordance with the process described in UnitedStates application Serial No. 95,587, now United States Patent2,592,616, filed in the names of Louis L. Stott and Laurence R. B.Hervey on May 26, 1949; United States application Serial No. 273,566,now U. S. Patent 2,742,440, tiled in the names of Louis L. Stott andLaurence R. B. Hervey on February 26, 1952; and the United Statesapplication Serial No. 202,405, now United States Patent 2,639,278,.1edin the names of Louis L. Stott and Laurence R. B. Hervey on December 22,1950. These methods disclose that nylon may be dissolved in mixtures oflower alcohols and water or in methanol alone under pressure and atelevated temperatures, and that nylon may be dissolved in polyhydricalcohols merely by heating them together. Oxygen is excluded during theheating step. Upon cooling, the nylon precipitates as a linepowder'which, when washed and dried, is suitable for the presentprocess. `In the case of polyhydric alcohols cooling may conveniently beaccomplished by adding water to the hot nylon solution. If waste nylon.is employed, undissolved material is preferably removed when the polymeris in solution.

lt has been found preferable to use nylon which has been prepared in themanner just described. This may be because of the diiculty in obtainingmaterial having an average ultimate particle size of less than 40microns as is obtained by the process described. It is also to berecognized that the product obtained by precipitating nylon as describedin the above-identified application is crystalline in character andbecomes more so upon sintering as disclosed by X-ray diffractionpatterns. But ,for whatever the reason, the nylon must be reduced in`size to the order of less than 40 microns in diameter.

I have found that contrary to established procedures for handlingthermoplastic materials it is possible to cold press thepolyamide-:carbon black mixture, remove the article from the mold andsubsequently sinter finely divided polyamide materials into finishedarticles, provided the starting particle sizes are of the orderindicated. No precautions need be observed with respect to the type ofmetals coming in contact with the nylon. Although carbon black may beadded in amounts of from 5 percent up to 85 percent by bulk volume ofthe nylon-ller mixture to advantage, a preferable carbon blackconcentration range is from about to 70 percent of the total bulkvolume. However, lower percents of carbon black also give finishedmolded articles of good strength.

The use of carbon black as a filler serves to reduce hygroscopic orthermal expansion as compared to articles made entirely of nylon, and atthe same time aids materially in securing dimensional accuracy throughreduced distortion during sintering. In addition, carbon black-nylonmixtures exhibit better fill ratios than that shown by the iinelydivided nylon alone. Thus, flll ratios Ias low as 3 have been attainedwith a carbon blacknylon mixture compared to about 4 for nylon alone formaterial with suitable strength after sintering. Since this till ratiorepresents the ratio of volume of material which must be put into a moldto the nal volume of the molded article, it will be seen that thelowerthe fill ratio, the less volume of molding composition is requiredand the smaller the mold which can be used.

To incorporate the carbon black, it is mixed with the nylon powder; itmay be added to the polyamide raw material, the polyamide dissolved inan agent which is a swelling agent for the polymer at elevatedtemperatures and a non-swelling agent at room temperature, and the twoprecipitated by cooling; the carbon may be added to the polyamidesolution before precipitation; it may be added to the precipitatedpolyamide while dispersed in said agent; after precipitating the carbonmay be added and thoroughly mixed with the wet, drained polyamide beforeor after washing; or the carbon may be added to the dried polyamidewhich has subsequently been wetted. In all cases the carbon is added tothe polyamide when Wet. The mixture of nylon powder with carbon black isbriquetted by the use of sufiicient pressure to .cause the resultantshaped article to withstand moderate shocks incident to its handling.The pressures employed range generally between about l0 tons per squareinch and 50 tons per square inch. The pressures do not seem to becritical, but it has been found that about 25 tons per square inch is avery satisfactory pressure. Pressure in the order of 3 tons per squareinch yields a briquette which ymay be handled only with considearblecare and when red has a compressive strength of less than onehalf thatof a similar piece pressed at 25 tons per square inch. Pressures inexcess of 75 tons per square inch are not required. The resultant coldpreformed article is then sintered by heating it under non-oxidizingconditions to a temperature below the melting point of the nylon presentfor a time sutlicient to cause the article to be strong and hard whencooled. In accordance with application Serial No. 272,966, tiledFebruary 23, 1952, in the name of Louis L. Stott, I have found thatifLseveral pieces are molded from nylon powder at room temperatureandsintered at various temperatures ranging from slightly above roomtemperature up to nearly the melting point and the resultant pieces aretested for compression strength, that an interesting phenomenon becomesapparent. Referring now to the drawing, the curves are plots of theloads required to break bearings 1 inch long, 1/2 inch I. D. and 1% inchO. D. when the loads are applied along the surface perpendicular to theaxis. Curve A is a plot of epsilon-caprolactam polymer sintered atvarious temperatures; curve B is al plot of the loads required to breaksimilar bearings of polyhexamethylene sebacamide sinteredat varioustemperatures; and curve C is a plot of the loads required tobreakisimilar bearings of polyhexamethylene adipamide at varioustemperatures. The bearings were sintered in vacuo.

It will be noted that in the accompanying drawing,the log of thecompressive load is plotted against the reciprocal of the temperature indegrees absolute (degreesicentigrade +273". Since two substantiallystraight lines result from plotting increasing sintering temperaturesfor each material, it is apparent that below the inflection point on anycurve a single process is operative which is a function solely of anactivation energy and the temperature. Above the inection point a newprocess is operative with a different activation. It is this secondprocess which 'forms part of this invention since sintering below theinflection point is ineffective. This inflection point is displayed notonly for the pure polyamides but also for mixtures containing carbonblack.

It is not known why the strength suddenly begins to increase at a rapidrate, but the inflection point may be easily determined for any givenpolyamide. Reference will be made hereafter to the break in the curve asthe inflectionpoint. The temperature to which the polyamide should beheated is therefore above the inection point and below the point whereany substantial molten phase occurs. If any substantial amount of moltenphase occurs, the article warps, blisters and becomes unusable. As maybe observed, the temperature to which the carbon black mixturescontaining polyhexamethylene adipamide should be sintered is from about200 C. to about 263 C., those containing polyhexamethylene sebacamide isfrom about 190 C. to about 220 C., and those containing the polymer ofepsilon-caprolactam is from about C. to about 215 C.

Before molding the powder mixture it may be granulated, if desired, toobtain freer flowing powder. Molding is advantageously accomplished byplacing the granulated powder mixture in a mold or otherwise compressingit as by passing it through pressure rollers.

After forming, the cold preformed nylon-carbon black article is thensintered by heating it under non-oxidizing conditions. The time range isusually between 2 to 30 minutes.

The presence of moisture in the nylon powder can,

under some conditions, cause cracks to appear in the article onsintering. This is particularly true when the sintering is done in hotoil as contrasted with sintering in vacuo. It has been found thatrelatively small bearings containing 3 percent moisture or more beforesintering, will crack if immersed directly in hot oil. On the other hand,a similar bearing rst immersed in cold oil and then the oil raisedslowly to the sintering temperature will be free from cracks. It istherefore preferred to keep the moisture content of the formed nylonarticle before sintering as low as possible, preferably below about lpercent moisture. In the case of large solid objects, the presence ofmoisture is more critical and a vacuum drying step either on the powderor the preformed article is desirable.

In the above discussion the pressing has been assumed to take place atabout room temperature. If the temperature of the mold is raised to apoint close to the melting point of nylon, very unsatisfactory materialis produced. A somewhat elevated temperature may be tolerated, however,and such temperature appears to add some strength to the green article,but does not materially alfect the finished product when sintered. It isessential therefore that the pressing ibe accomplished at a temperaturebelow that at which any molten phase can occur and preferably at orabout room temperature.

The reason for the extremely tight bond obtained by my process is notapparent. It may be that the high pressure employed, followed bysintering, causes suflicient reorientation analogous to crystal growthto secure adequate bonding. But whatever the reason may be, it istotally unexpected that a strong bond would form, and it is unexpectedthat any amounts of carbon black up to 85 percent of the total bulkvolume yield articles of surprising strength.

A further unexpected advantage is secured by my technique in that powderobtained from waste textile nylon, such as stocking material, may besuccessfully used either alone, or in conjunction with powder obtainedfrom virgin nylon. All attempts to melt and mold or extrude waste nylonhave resulted heretofore in excessively brittle products not suitablefor commercial use. It is apparent, therefore, that the high costfactors restricting the wider use of molding nylon shapes have beenlargely overcome by the process described herein. The special andexpensive tools required by the present techniques are replaced by therelatively simple and available cold pressing equipment used in thepowder metallurgy art and the relatively expensive virgin nylon may besubstituted in whole, or in part, by nylon obtained from nylon scrap andtextile waste, such as textile clippings and stockings.

If close tolerances are not required, the pressed, sintered articles maybe satisfactorily used without -subsequent conditioning or annealing toremove strains which may occur to a minor extent in the molded articles.In some bearings and other articles, the necessity foryclose tolerancesrequires that the article be free from slight additional shrinkages inservice. Annealing is accompli-shed by simply heating the article,preferably under non-oxidizing conditions, to a temperature under, butpreferably close, i.e., within 50 C. to the melting point of thepolyamide for a few minutes to an hour and slowly cooling. Holding forlonger times at lower temperatures also reduces strains. In some cases,annealing may be combined with the sintering step to avoid two furnacetreatments.

If desired, lubricating agents in minor amounts may Ibe added tofacilitate and improve uniformity of the shaped piece and to aid theremoval of the pressed piece from the die or for other purposes. Usefullubricants include stearates such as zinc stearate, hydrogenated cottonseed oil, or other greasy or soapy substances which may be eitherintermixed with the carbon filled nylon powder or coated on the diesurfaces. Small amounts of other minor ingredients may be added toimprove the flowing proper- 6 ties of the dry powder or to aid inobtaining uniform physical properties in the sintered product.

One of the important uses for pressed and sintered nely divided nyloncontaining carbon black has proved to be in the field of wear resistingparts such as bushings and bearings. Polyhexamethylene adipamide andcarbon black, in particular, have exhibited considerable merit forbushings operating at light loads, especially where good strength andwear resistance is required. In many instances these bushings have shownsuperior wear resistance to similar pieces made from powdered bronze.The bearings made in accordance with this process are fully equal to anynylon bearings made from nylon by any of the present standard techniqueswith respect to being free from any gross amount of strain. Tests haveindicated that the tendency to seize is less than that of injectionmolded bearings and therefore theymay be made to closer tolerances.Other articles which may be advantageously prepared by the process ofthis invention include various small, irregular shapes employed wherethe wear resistance and strength are important factors. Such aticlesinclude, but are not limited to, small rollers, cams, valve seats,gears, bushings, etc., and articles requiring good strength andwear-resisting characteristics.

As many apparently widely different embodiments of this invention may bemade without departing from the spirit and scope thereof, it is to lbeunderstood that I do not limit myself to the specicembodiments thereof,as illustrated by the following examples, except as defined in theappended claims.

Example I Two hundred and forty grams of virgin polyhexamethyleneadipamide molding powder was dissolved in 1500 grams of ethylene glycolin a carbon dioxide atmosphere at 193 C. Six hundred and thirty-threegrams of carbon black was added to this hot solution (equivalent to 66percent carbon black byvolume) and the solution was allowed to cool to155 C. when it was then quenched with an excess yof cold water. Theco-precipitated carbon black-polymer mixture was then washed to removethe ethylene glycol and dried to give a ne powder with a fill ratio of3.25 as compared with about 4 for the polymer of hexamethyleneadiparnide alone.

Example II A portion of the powdered mixture made in Example I waspressed at room temperature at a pressure of 25 tons per square inch togive bushings with an inside diameter of about 0.5, an outside diameterof 0.75 and 0.75" long. The bushings were removed in a green conditionand one was heated in a vacuum to 263 C. while another was placed in abath of Meprolene (a temperature-resistant hydrocarbon oil) heated to263 C. After the bushings were heated throughout, they were removed andcooled. Breaking thrusts, determined by applying loads perpendicular tothe long axes of the bushings, for the green bushings and for thosesintered in vacuum and in Meprolene were 65, 175, and pounds,respectively.

Example III Three hundred sixty grams of polyhexamethylene adiparnidewas dissolved in 2500 grams of ethylene glycol in a carbon dioxideatmosphere at 193 C. Three hundred ninety grams of carbon black wasadded to this hot solution (equivalent to 40 percent carbon black byvolume). The solution was quenched at C. and the co-precipitated powderwashed and dried as described in Example I. The powder had a fill ratioof 3:1.

Example l V Bushings were made up from the powder of Example III asdescribed in Example II. Breaking thrusts for the green bushings .andfor those sintered in vacuum Iand in Merprolene were 45, 125, and 120pounds respectively.

7 Example V Th1-ee hundred .sixty grams of polyhexamethylenek adipamidewas dissolved in 1500 grams of ethylene glycol in a. carbon dioxideatmosphere at 193 C. One hundred forty-seven grams of carbon black wasadded to this hot solution (equivalent to y20 percent carbon black byvolume). The solutionwas quenched at 155 C. andthe co-precipitatedpowder washed and dried'as described in Example I. The powder had a llratio of 3.0.

Example VI black, said linear polyamide being a polyamide which isvsoluble in phenol at room temperature and insoluble in ethylene glycolexcept `at temperatures abovel about 140 C., and having an ultimateparticle size of less than 40 microns, said carbon black being presentin a concentration of 5 to 85 percent by bulk volume.

2. A composition in accordance with claim 1 wherein the carbon black ispresent in a concentration of 20 to 70 percent by bulk volume.

3. A composition having a low fill ratio comprising a mixture of finelydivided linear polyamide and carbon black, said linear polyamide being apolyamide which is soluble in phenol at room temperature and insolublein ethylene glycol except at temperatures above about 140 C., saidmixture having been formed by adding saidl rcarbon black to a solutionof said polyamide in ethylene glycol, cooling to co-precipitate theresulting mixture of polyamide and carbon black, and` washing and dryingsaid co-precipitated mixture, said carbon black being present in aconcentration from 5 to 85 vpercent by bulk volume.

4. A shaped polyamide article comprising sintered synthetic linearpolyamide particles and carbon black, said polyamide being one which issoluble in phenol at room temperature and insoluble in ethylene glycolexcept at temperatures above about 140 C., and having an averageultimate particle size of less than 40 microns, said carbon blackconstituting from 5 to 85 percent by total bulk volume of said article,said article being characterized by being bonded by sintering actionalone.

References Cited in the le of this patent UNITED STATES PATENTS2,345,533 Graves Mar. 28, 1944 2,592,616 Stott et al Apr. 15, 19522,639,278 Stott et al May 19, 1953 2,695,425 Stott Nov. 30, 19542,698,966 Stott et al. Ian. 11, 1955 2,748,099 Bruner etal. May 29, 1956

1.A COMPOSITION HAVING A LOW FILL RATIO COMPRISING A MIXTURE OF FINELYLINEAR POLYAMIDE AND CARBON BLACK, SAID LINEAR POLYAMIDE BEING APOLYAMIDE WHICH IS SOLUBLE IN PHENOL AT ROOM TEMPERATURE AND INSOLUBLEIN ETHYLENE GLYCOL EXCEPT AT TEMPERATURES ABOVE 140* C., AND HAVING ANULTIMATE PARTICLE SIZE OF LESS THAN 40 MICRONS, SAID CARBON BLACK BEINGPRESENT IN A CONCENTRATION OF 5 TO 85 PERCENT BY BULK VOLUME.